The present invention relates to a mask with a phase shifter used for the formation of fine patterns in various solid state devices such as semiconductor devices, superconductor devices, magnetic devices, SAW devices and OEIC's (opto-electronic IC's), a method of manufacturing the mask and a method of forming a pattern by use of the mask.
Conventionally, the formation of fine patterns for solid state devices such as VLSI's has mainly been made by a reduction projection system. In the reduction projection system, an optical projection system is used to form an image of a mask pattern on a resist-coated substrate, thereby transferring the mask pattern. The resolution limit in the reduction projection system is proportional to an exposure wave length .lambda. and inversely proportional to a numerical aperture NA of the optical projection system. Accordingly, the improvement of the resolution has been promoted by shortening the exposure wave length and increasing the NA of a projection lens. However, the improvement of the resolution relying on this method is nearing a limit in the aspects of the design of the optical projection system, the manufacturing technique, the restriction on a light source, and so on.
On the other hand, as one method of overcoming such a limit there exists a method (hereinafter referred to as a phase shifting method) in which the improvement of the resolution and the increase of the depth of focus are intended by introducing a phase shift between lights passed through adjacent transparent portions on a mask. According to this method, for example, in the case of a repeating pattern of elongated transparent and opaque areas, transparent materials (hereinafter referred to as phase shifters) for introducing a phase shift are provided for every other transparent area so that a phase shift between lights passed through adjacent transparent areas on the mask becomes substantially 180.degree.. JP-A-58-173744 (corresponding to U.S. Ser. No. 365,672 filed on Apr. 5, 1982 now abandoned) and IEEE, Trans. Electron Devices, ED 29, No. 12 (1982), pp. 1828-1836 have disclosed that the resolution in the case of the above pattern is improved by about 40% as compared with a method in which no phase shifter is provided.
A resist material and a spin on glass (SOG) material are each known as a material which forms the phase shifter. The resist material has a demerit that the mechanical durability thereof is weak.
A mask manufacturing method in the case where the SOG material is used as the phase shifter forming material, includes a step of forming an antistatic layer or film of indium tin oxide on a glass plate substrate, a step of forming a chromium film of a predetermined configuration on the antistatic layer, a step of thereafter spin-coating an SOG film on the substrate, a step of forming a resist film of a predetermined configuration on the SOG film and a step of etching the exposed SOG film.
In the above prior art, the phase shifter consists of one layer. Therefore, especially when the phase shifter layer is formed by a spin coat film, the fluctuations of thickness of the spin coat film caused by the unevenness of the underlying surface give rise to trouble. An opaque or light shielding film used in a usual mask includes a Cr film and has a film thickness of about 80 nm. This film is disposed with a variety of configurations and pattern densities. When a phase shifter layer of a spin coat film is formed on the Cr film, the phase shifter layer surrounded by the Cr film is formed with a film thickness which is larger by the thickness of the Cr film as compared with the thickness of the spin coat film. However, in the case where the most of the underlying surface is occupied by a transparent portion and hence the Cr film is formed over a small area, the phase shifter layer is formed with a film thickness which is substantially the same as the thickness of the spin coat film. For example, in the case where a fine pattern of Cr film is repeatedly arranged beside a large pattern of Cr film, the film thickness of the phase shifter layer on the transparent portion beside the large pattern of Cr film is larger by about 80 nm than the thickness of the spin coat film but the film thickness of a portion of the phase shifter layer apart from the large pattern of Cr film is nearly the thickness of the spin coat film. Thus, a macroscopical thickness distribution is developed in the mask. Accordingly, the film thickness of the phase shifter layer has the fluctuations which may locally amount to about 80 nm. Even if the phase shifter layer is preliminarily spin-coated with a small thickness of 40 nm with consideration for such an error in film thickness, the fluctuations of 40 nm cannot be prevented. The phase shifter film thickness to be set is different depending upon the refractive index of the film. For example, in the case where the exposure wave length is about 365 nm and the refractive index of the film is 1.45, the optimum film thickness for the phase shifter layer is about 406 nm and hence the error of 40 nm amounts to about 10%. The version of this film thickness error calculated in terms of a phase shift assumes an error of about 20.degree. which may greatly affect the pattern resolution.
Also, the above prior art has a problem that the method can be applied to only a certain specified pattern configuration such as the repeating arrangement of transparent and opaque areas. Further, in the above prior art, a plurality of transparent areas between which the phase shift of 180.degree. is introduced must be separated from each other. In the case where such a separated pattern is to be formed, there is a problem that a negative tone resist material must be used or it is difficult to use a positive tone resist material which is mainly used in the existing LSI fabrication process.
Furthermore, in the above prior art, there is an intricate problem that a resist material must be used for delineating the spin on glass layer.
Moreover, when a phase shifting mask is fabricated by virtue of the above prior art, the following problem is encountered in (1) the case where a phase shifter is not formed in a predetermined area by the phase shifting mask or the phase shifter has a deficiency, (2) the case where a phase shifter is formed outside a predetermined area and is left or (3) the case where the film thickness or the like of a phase shifter is different from a desired value though the phase shifter is formed at a predetermined location. Namely, the whole of the defective phase shifter should be removed and a new phase shifter should be thereinstead formed again or a new mask should be fabricated again from the beginning, which requires labors and a time.
U.S. Pat. No. 5,235,400 filed on Oct. 10, 1989 and assigned to the assignee of the present application has disclosed the detection of a defect in a photomask in which the defect is detected on the basis of the intensities of the transmitted version and/or reflected version of light with which the mask having the phase shifter is irradiated.